The term "rotary atomizer" refers to a type of liquid spray coating apparatus which includes an atomizer head rotatable at high speed (typically 10,000-40,000 rpm) to effect atomization of a liquid coating material to be applied to a workpiece. The head is usually in the form of a disk or a cup which includes an interior wall defining a cavity and terminating in an atomizing edge. Liquid coating material delivered to the interior of the cup migrates outwardly under centrifugal force along the wall until it is flung from the edge of the cup and thereby atomized. To improve the transfer efficiency of the coating process it is normally desirable to impart an electrostatic charge to the coating material to attract the atomized coating material to an electrically grounded workpiece. An example of an electrostatically charged type rotary atomizer is disclosed in commonly assigned U.S. Pat. No. 4,887,770 to Wacker et al. which is expressly incorporated herein by reference in its entirety. As the foregoing patent also recognizes, transfer efficiency can be further improved by providing a plurality of air jets surrounding the cup to shape the cloud of atomized material and propel it toward the workpiece. In order to facilitate rapid and efficient changing from one color or type of coating material to another, the Wacker et al. '770 patent and the patents discussed below teach providing a valve for selectively flushing the cup and the line which feeds coating material to the cup with solvent in order to clean that line and the cup prior to changing colors or types of coating material.
U.S. Pat. No. 4,422,576 to Saito et al. discloses a color change apparatus and method for an electrostatic rotary atomizer wherein a pair of color change valve manifolds are located remotely from the rotary atomizer. Each manifold includes a plurality of individual color valves whose outlets are connected to a common material feed line as well as valves for selectively delivering paint thinner and air into the feed line for flushing. The inlet of each color valve is connected to a supply of coating material of a particular color. Each of the common feed lines is connected to a first change-over valve mounted to the rotary atomizer in the high voltage region adjacent the rotary atomizing head. The first change-over valve selectively couples one of the feed lines either to the rotary atomizing head or to an inlet of an adjacent change-over valve by way of a first drain line. The second change-over valve includes another inlet connected to a second drain pipe as well as an outlet connected to a third drain pipe. The second drain pipe communicates with a shroud surrounding the atomizing head while the third drain pipe runs to a remotely located ejector valve.
To change from one color to another, solvent is fed at high pressure and at a great flow rate together with bursts of air through: the feed line, the first change-over valve, the first drain pipe, the second change-over valve and finally to the ejector by way of third drain pipe. Then, the change-over valves are shifted to feed solvent to the atomizing head from which it collects in the shroud. The shroud drains via the second drain pipe and through the second change-over valve to the ejector valve by way of the third drain. Prior to introducing the next color of coating material into the feed line, air and thinner under high pressure are flushed through the third drain line by way of the first change-over valve, first drain line and second change-over valve prior to reapplying high voltage to the rotary atomizing head.
This system suffers from the drawback of requiring flushing of the long paint lines between the color changer and the change-over valves. This increases both wastage of coating material and the amount of solvent required to flush the system sufficiently to avoid contaminating the next desired coating material with the color used previously. Rapid and complete flushing is further inhibited owing to the circuitous coating material path which must be flushed. That path is not only long and voluminous but also includes areas of irregular shape and changing cross-section where coating material will tend to accumulate.
U.S Pat. No. 4,380,321 to Culbertson et al. discloses a color change valve structure for a rotary atomizer which includes a coating material valve and a dump valve. Both valves are mounted in a single valve body which is located just behind the rotary atomizing head. The coating material valve includes an inlet which is connected to a feed line carrying either coating material or flushing media from remotely located valves. The dump valve operates to selectively connect the feed line through the material valve to a dump outlet. To purge the system of material of a first color in preparation for spraying material of a different color, the dump valve is opened and a flow of flushing media is established through the supply line to cleanse the supply line and material valve and expel waste through the dump outlet. Thereafter, the dump valve is closed and the material valve is momentarily opened to cleanse that portion of the coating material supply path located between the material valve and the atomizing head to prepare for spraying material of the different color.
While the proximity of the coating and dump valves to the atomizing head in this arrangement reduces the quantity of coating material and flushing media impinging on the spray head during color changing, this system does not eliminate the need to flush a long feed line connected to the material valve from a remotely located color changer. Moreover, like Saito et al. '576 the first and second change-over valves and their interconnections with the atomizing head and with one another define a circuitous flow path which presents many areas in which material can accumulate and which is therefore difficult to flush thoroughly.
Accordingly, there is a need for a rotary atomizer which is capable of spraying a plurality of colors without requiring the flushing of long coating material feed lines between the rotary atomizer and a common coating material feed line for supplying colors one at a time to the atomizer. There is also a need for such a rotary atomizer wherein the portion of the coating material path which must be flushed is kept to a minimal volume and presents few, if any, regions wherein coating material may accumulate in pockets which are difficult to flush thoroughly.
Another drawback of the prior art concerns problems in controlling the flow rate of coating material expelled from the atomizing head. Since flow rate is correlated to pressure, it has been known to connect a fluid pressure regulator in series with the coating material supply line connected to an atomizer. Such regulators have heretofore been mounted outside the housing of the rotary atomizer in series with the coating supply line. Applicants have recognized that this is undesirable for a number of reasons.
First, accurate flow control is facilitated if the pressure adjacent the nozzle supplying coating material to the atomizing head is substantially the same as the pressure at the outlet of the regulator. Locating the regulator exteriorly of the atomizer increases the pressure drop between the regulator and the nozzle supplying coating material to the atomizing head thereby degrading the accuracy of control. Control response time also suffers from such mounting since there can be a significant time lag between a change in pressure at the outlet of the regulator and a corresponding flow change at the outlet of the nozzle which delivers coating material to the atomizing head. These control problems are exacerbated in rotary atomizers such as Saito et al. and Culbertson et al., supra both of which include valving immediately upstream of the atomizing head in the coating material supply path. Such valving increases the length of the mean flow path and gives rise to an even larger pressure drop.
Positioning a pressure regulator remotely from the atomizer causes even further pressure regulation inaccuracy when a rotary atomizer is to be mounted on an oscillator having significant vertical travel. As the atomizer is raised and lowered, the pressure head between the regulator and the nozzle supplying the coating material to the atomizing head will vary as a function of the vertical height of the atomizer. As a result, the atomizer will tend to deliver less coating material when raised as compared to when the atomizer is in a lower position.
While mounting a regulator remotely of the atomizer offers an advantage in that the regulator can be conveniently disconnected when not required for a particular job, such external mounting is bulky and exposes the regulator to overspray and other contaminating build up particularly where the regulator is mounted inside a spray booth.
Accordingly, there is a need for a rotary atomizer which is compact and which includes a fluid pressure regulator mounted for improved flow rate control accuracy and improved response time, irrespective of the vertical position of the rotary atomizer. There further exists a need for a rotary atomizer offering such improved control while protecting the regulator from a contaminating environment and yet permitting the regulator to be conveniently disconnected and reconnected depending on the requirements of a particular coating job.